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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration
3
Plutonium-238 Supply
This chapter addresses NASA’s plutonium-238 (238Pu) needs and how they can be satisfied.
FOREIGN OR DOMESTIC 238Pu?
When U.S. nuclear weapons production facilities were shut down in 1988 and subsequently decommissioned, the United States lost the ability to produce 238Pu (except for very small amounts for research). The substantial cost of maintaining those facilities could not be justified solely on the basis of producing 238Pu, especially given the large 238Pu stockpile that existed at the time. That stockpile was sufficient to support radioisotope power system (RPS) missions through the 1990s and into the early 2000s.1 To supplement the Department of Energy’s (DOE’s) dwindling stockpile of 238Pu, the DOE executed an agreement with Russia in 1992 to purchase 238Pu from Russia. The DOE has taken delivery of 20 kg to date. There are three more orders to be delivered, totaling less than 20 kg.2
To the best of the committee’s knowledge, 238Pu is no longer being produced in Russia (or anywhere else), and there is not a substantial amount of 238Pu left in Russia (or anywhere else) available to meet NASA’s needs, beyond that which Russia has already agreed to sell to the United States. Purchasing 238Pu was intended as a stopgap measure until U.S. production was reestablished, and continued procurement from Russia cannot serve as a long-term solution to U.S. needs unless Russia itself reestablishes a 238Pu production capability. Such a move would require a major investment in Russian production facilities—an investment that Russia seems unlikely to make unless the United States pays for it.
Restarting production of 238Pu in Russia would take longer than restarting domestic production because of the long time it would take to negotiate an agreement with Russia and to complete the National Environmental Policy Act (NEPA, 1970) process, which would apply to Russian production of 238Pu if it were funded by the U.S. government. Based on prior experience, it would probably take 2 or 3 years just to negotiate and finalize an agreement with Russia before work could begin. In addition, 238Pu obtained from Russia can be used only for civil applications and cannot be used to satisfy U.S. national security applications, should they arise. Russia has agreed to sell 238Pu to the United States with the limitation that it be used only for peaceful space missions, and that same stipulation would presumably apply to future purchases.
A similar situation would likely exist if the United States attempted to obtain 238Pu from a nation other than Russia: a large capital investment would be needed to construct new facilities and/or refurbish existing facilities; the work would need to comply with NEPA if it were funded by the United States; and the long time necessary to negotiate an agreement, obtain funding, and start work would create a substantial shortfall in 238Pu available for NASA missions.
FINDING. Foreign Sources of 238Pu. No significant amounts of 238Pu are available in Russia or elsewhere in the world, except for the remaining 238Pu that Russia has already agreed to sell to the United States. Procuring 238Pu from Russia or other foreign nations is not a viable option.
HOW MUCH DO WE NEED?
On April 29, 2008, the NASA administrator sent a letter to the secretary of energy with an estimate of NASA’s future
1
Because of radioactive decay, 238Pu cannot be stored indefinitely. However, with a half-life of 88 years, 238Pu decays rather slowly. After a storage period of 20 years, 85 percent of the original amount will still remain.
2
The Department of Energy did not provide an exact estimate of how much 238Pu it expects to have on hand after the deliveries of Russian 238Pu are complete. Based on available information, the committee estimates that there will be a total of approximately 30 kg of 238Pu available for NASA, including the 238Pu that has already been used to fuel the RPS for the Mars Science Laboratory, whose launch date has been postponed from 2009 to 2011.
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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration
TABLE 3.1 NASA’s Demand for 238Pu, 2009-2028 (as of April 2008)
238Pu (kg)
Mission
Launch Date
Watts
Type of Radioisotope Power System
3.5
Mars Science Laboratory
2009a
100
MMRTG
1.8
Discovery 12/Scout
2014
250
ASRG
24.6
Outer Planets Flagship 1
2017
600-850
MMRTG
3.5
Discovery 14
2020
500
ASRG
5.3
New Frontiers 4
2021
800
ASRG
14
Pressurized Rover 1
2022
2000
High-performance SRGb
14
ATHLETE Rover
2024
2000
High-performance SRG
1.8-5.3
New Frontiers 5
2026
250-800
ASRG
3.5
Discovery 16
2026
500
ASRG
14
Pressurized Rover 2
2026
2000
High-performance SRG
5.3-6.2
Outer Planets Flagship 2
2027
700-850
ASRG
14
Pressurized Rover 3
2028
2000
High-performance SRG
105-110
Total demand for 238Pu, 2009-2028 (kg)
5.3-5.5
Annual demand (20-year average in kg/year)
NOTE: ASRG, Advanced Stirling Radioisotope Generator; ATHLETE, All-Terrain Hex-Legged Extra-Terrestrial Explorer; MMRTG, Multi-Mission Radioisotope Thermoelectric Generator; SRG, Stirling radioisotope generator.
aThe launch date for the Mars Science Laboratory mission is currently 2011.
bA high-performance SRG is a yet-to-be-developed concept that would use ASRG technology to meet the high power requirements of the lunar rovers.
SOURCE: Letter from the NASA administrator Michael D. Griffin to secretary of energy Samuel D. Bodman, April 29, 2008 (reprinted in Appendix C).
demand for 238Pu.3 The committee has chosen to use this letter as a conservative reference point for determining the future need for RPSs (see Table 3.1). However, the findings and recommendations in the report are not contingent upon any particular set of mission needs or launch dates. Rather, they are based on a conservative estimate of future needs. The estimate of future needs is also consistent with historic precedent. For example, the mission set described in the administrator’s letter is consistent with the mission set in the current Agency Mission Planning Model, although the latter includes three additional RPS-powered missions: two International Lunar Network missions (that could be launched in 2013 and 2016) and a Mars Lander mission (that could be launched in 2016). These additional missions are not included in Table 3.1, but the total amount of 238Pu required to fuel these additional missions is estimated to be 3.6 kg or less. As noted below, even if the 238Pu required by these missions is not considered, the DOE should take immediate action to reestablish domestic production of 238Pu. Including the International Lunar Network and Mars Lander missions in the demand estimate would only increase the projected 238Pu shortfall.
The administrator’s letter requests that the DOE maintain the capability to provide NASA with fueled RPS assemblies for 12 missions during the 20-year period from 2009 to 2028. These missions have electrical power requirements ranging from 100 to 2,000 watts (see Table 3.1).
The amount of 238Pu required to meet the needs of these 12 missions will depend upon the type of RPS used to convert the thermal energy of the 238Pu fuel to electrical energy. The Mars Science Laboratory is equipped with a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), and the MMRTG is also currently baselined for use on the Outer Planets Flagship (OPF) 1 mission. As Chapter 4 describes in more detail, this is the only type of RPS that is currently available, and it has a low energy-conversion efficiency (of just 6.3 percent). The Advanced Stirling Radioisotope Generator’s (ASRG’s) energy conversion efficiency is predicted to be 28 to 30 percent, and an ASRG will produce more electricity than an MMRTG even though it will be powered by just two general purpose heat source (GPHS) modules instead of the eight modules used by an MMRTG.
The ASRG or some other type of Stirling radioisotope generator is baselined for all other missions listed in the administrator’s letter.4 All 12 missions will require a total of 105 to 110 kg of 238Pu, which is equivalent to an average production rate of 5.3 to 5.5 kg per year for 20 years.
3
Letter from the NASA administrator Michael D. Griffin to secretary of energy Samuel D. Bodman, April 29, 2008 (reprinted in Appendix C). During the late 1980s and early 1990s, NASA periodically sent similar letters to DOE to update DOE regarding NASA’s requirements for 238Pu.
4
As described in Chapter 4, the International Lunar Network missions, if they take place, would likely be powered by a third type of RPS: a yet-to-be-defined “Small RPS.”
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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration
PLUTONIUM-238 PRODUCTION PROCESS
Production of 238Pu is a complex process. At the top level, this process involves the following steps:
Processing of materials prior to irradiation.
Purify neptunium-237 (237Np).
Fabricate 237Np targets.
Irradiation of targets in a nuclear reactor to transform 237Np into 238Pu.
Processing of materials after irradiation.
Extract, separate, and purify 238Pu and the remaining 237Np from the irradiated targets.
Recycle the extracted 237Np so that it can be used to make more targets.
Process the 238Pu so that it can be used to fabricate RPS fuel pellets, which are then assembled into GPHS modules.
The capabilities of existing facilities and the expertise of existing staff at the DOE’s Idaho National Laboratory (INL) and Oak Ridge National Laboratory (ORNL) make them the best places to carry out the above steps. In particular, there are just two operational reactors in the United States that can enable the production of large amounts of 238Pu (on the order of kilograms per year) in a timely fashion: the Advanced Test Reactor (ATR) at INL and the High Flux Isotope Reactor (HFIR) at ORNL.
The ATR and HFIR reactors are light-water fission reactors that use enriched uranium as fuel. Both have numerous cylindrical voids at various locations in and around the reactor core where targets can be inserted and irradiated. The rate at which 237Np is transformed into 238Pu will vary greatly according to the location of the 237Np targets in the reactor.
There are nine primary test positions (flux traps) in the ATR.5 Six of these are dedicated full-time to the DOE’s Office of Naval Reactors. This office is responsible for developing reactors to power submarines and aircraft carriers for the U.S. Navy. Naval Reactors is the primary customer for the ATR and the primary source of funds used to sustain the ATR.
There are also many other usable positions in the ATR where 237Np targets could be irradiated, although the outer positions have neutron and gamma fields that are an order of magnitude lower than the positions nearest the center of the core. If 237Np targets are placed in all of the core positions except for the six flux traps that are dedicated to Naval Reactors, ATR is thought capable of creating up to 4.6 kg of 238Pu per year using proven, cylindrical 237Np targets and standard reactor operating conditions. Advanced targets with a more complex geometry, which could be introduced later as a process improvement, would increase the yield, perhaps as high as 5.8 kg/year. A yield of 3 to 4 kg/year would allow ATR to produce 238Pu while still supporting the Office of Naval Reactors as well as other users, such as the National Scientific User Facility.
Like the ATR, HFIR also has multiple positions where targets can be irradiated. The DOE’s Office of Science is HFIR’s primary user. Assuming that HFIR will continue to support its primary mission of neutron science, HFIR can create, at most, about 2 kg/year of 238Pu using standard target designs and reactor operating conditions. However, this would reduce the amount of support that it can provide to secondary activities, such as production of medical and industrial isotopes.
Some test positions tend to produce unacceptably high concentrations of an unwanted Pu isotope (236Pu) in irradiated targets. Unlike 238Pu, the natural decay of 236Pu produces significant gamma radiation, which makes handling and processing of irradiated targets much more difficult and hazardous. Because 236Pu has a half-life of just 2.9 years, if irradiated targets are determined to have too much 236Pu, they are stored until the 236Pu decays sufficiently so that radiation levels are within acceptable limits.
Ultimately, the total amount of 238Pu that the United States can easily produce is limited by the availability of 237Np. Trace amounts of 237Np occur naturally in uranium ores, but as a practical matter, 237Np used for 238Pu production must be artificially produced. 237Np is not currently being produced in the United States, and it would not be easy to restart production. (The existing stockpile was created as a byproduct of Cold War production of nuclear weapons material.) However, the United States has enough 237Np in storage at INL to produce 5 kg of 238Pu per year for more than 50 years.
Programmatic Options for Domestic Production
There are four primary options for initiating domestic production of 238Pu in a timely fashion. All of these options (1) rely exclusively on existing reactors (ATR and/or HFIR) to irradiate 237Np targets, (2) would require new or refurbished processing facilities to fabricate 237Np targets and extract 238Pu from the irradiated targets, and (3) would ship extracted 238Pu to Los Alamos National Laboratory for encapsulation in fuel pellets.6
5
Flux traps are areas with high levels of thermal neutron radiation, which is ideal for converting 237Np to 238Pu with minimal impurities.
6
The 238Pu encapsulation facilities at Los Alamos National Laboratory are currently operational and have been used to prepare fuel for past missions as well as the Mars Science Laboratory. All four programmatic options for domestic production of 238Pu assume that 238Pu encapsulation facilities will remain at Los Alamos National Laboratory because it would not be cost-effective to relocate them to another location such as INL.
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Option 1.
Use HFIR alone to irradiate 237Np targets, with processing of targets primarily at ORNL.
The HFIR, as currently configured, could yield 1 to 2 kg of 238Pu per year and still accommodate current, high-priority customers for that facility. If the HFIR were wholly dedicated to support 238Pu production—and if it were configured with a new beryllium reflector—the DOE estimates that it could yield at least 3 kg of 238Pu per year.7 However, like the ATR, the HFIR is a unique facility, and it is not realistic to expect that the DOE would displace all current users of that facility in order to dedicate the HFIR wholly to 238Pu production.
Option 2.
Use ATR alone to irradiate 237Np targets, with processing of targets primarily at INL.
It may be technically possible to get 5 kg/year from just the ATR, but only at the cost of displacing virtually all other users except for the Office of Naval Reactors, and at the cost of production flexibility when the ATR is out of service for routine or corrective maintenance.
Option 3.
Use ATR and HFIR to irradiate 237Np targets, with processing of targets primarily at INL.
If both the ATR and HFIR reactors are used to support 238Pu production, a yield of 5 kg/year could be achieved without displacing the primary customers of either facility, and 238Pu production would continue even when one of the reactors is shut down for routine or corrective maintenance. Under this option, 237Np targets would be fabricated at INL. Irradiation of 237Np targets would occur at both INL and ORNL. Plutonium-238 recovery and purification would occur at INL.
Option 4.
Use ATR and HFIR to irradiate 237Np targets, with processing of targets primarily at ORNL.
This option is the same as Option 3, except that the processing of targets before and after irradiation would be conducted primarily at ORNL. With this option, INL would continue to store the existing stockpile of 237Np, shipping it to ORNL as needed for fabrication of 237Np targets.
If and when the DOE is funded to reestablish 238Pu production, the DOE’s first task will be to decide which of the above options to use. The committee believes that both Options 3 and 4 are viable approaches for initiating domestic production of 238Pu, and the differences between these two options, in terms of cost, schedule, and so on, pale in comparison to the negative consequences of continued inaction to implement either option.
The major cost of implementing either Option 3 or 4 would be for capital improvements at the site where most of the processing operations would take place. For both approaches, previous, preliminary estimates by the DOE indicate that capital costs at the primary laboratory would be about $150 million over 5 to 7 years. The cost of capital improvements at the supporting center was estimated to be approximately $10 million to $12 million. The DOE will undoubtedly update these estimates as part of its site selection process. A reliable estimate of the incremental cost of producing each new kilogram of 238Pu, after capital improvements are completed, is not available.
Option 4 would allow fabrication of 237Np targets to start earlier than with Option 3.8 Thus, Option 4 would allow testing of targets in the ATR and HFIR reactors to start sooner than with Option 3. This testing is necessary to validate predictions regarding the yield of 238Pu and the presence of undesirable isotopes in targets irradiated at various locations in the reactors.
From 1998 to 2000, the DOE prepared a broad Environmental Impact Statement (EIS) concerning its nuclear facilities that included reestablishing 238Pu production in the United States. This EIS, entitled Final Programmatic Environmental Impact Statement for Accomplishing Expanded Civilian Nuclear Energy Research and Development and Isotope Production Missions in the United States, Including the Role of the Fast Flux Test Facility, is commonly referred to as the Nuclear Infrastructure Programmatic Environmental Impact Statement (NI PEIS) (DOE, 2000). This EIS established the need to produce 5 kg/year of 238Pu to meet national needs for RPSs. A record of decision was issued that approved the NI PEIS (Federal Register, 2001). To date, no Administration has requested and Congress has not provided funds necessary to implement the work described in the NI PEIS. The DOE could implement Option 3 or Option 4 using (1) a modification of an existing EIS for INL and (2) a separate existing EIS for ORNL (without modification).
In addition to the four options described earlier, other, less practical options also exist. For example, building a new reactor similar to HFIR or ATR would enable production rates substantially higher than 5 kg/year. This could completely eliminate 238Pu availability as a constraint on NASA missions and RPS designs. However, this approach would probably cost on the order of a billion dollars—much more than the cost of using existing reactors. In addition, it would probably take 10 to 15 years to complete the necessary reviews and construct a new reactor—too long to satisfy NASA’s future needs without a long hiatus in RPS-powered missions.
7
Most of the neutrons produced in fission reactors appear as high-energy (“fast”) neutrons. The beryllium reflector increases the rate at which fast neutrons slow down, thereby increasing the level of low-energy (“thermal”) neutron radiation in the reactor.
8
Options 3 and 4 would both require existing facilities to be upgraded. Option 3 would also require some new construction at INL before 237Np targets could be fabricated.
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Another approach would be to build multiple, large TRIGA (Training, Research, Isotopes, General Atomics) reactors,9 but the effectiveness of this approach has not been demonstrated. In any case, this option would take much longer than any option that uses the existing HFIR and ATR reactors, and it may not be possible to generate neutron flux levels in a TRIGA reactor high enough for useful 238Pu production rates.
It is also possible to produce 238Pu using a commercial light-water reactor (CLWR) operated by an electric utility. Such a reactor could yield 5 kg of 238Pu/year while still producing electricity. However, aluminum-clad 237Np targets, which have been used in the past and could be used with ATR and HFIR, would not be suitable for the high operating temperatures of a CLWR. Thus, this option would require development of new 237Np targets with Zircaloy or stainless steel cladding (DOE, 2000). It would take years to develop, test, and validate the performance of new target designs in specific locations in a particular commercial reactor. The Record of Decision for the NI PEIS notes that CLWR options for producing 238Pu “were not selected because of uncertainties in the target design, development and fabrication. The design and fabrication technology of neptunium-237 targets for irradiation in ATR and HFIR is much more mature” (DOE, 2001, p. 7887). Given that nothing has been done to address these uncertainties since the Record of Decision was issued in 2001, CLWRs are not a viable option for addressing the need to reestablish 238Pu production as soon as possible.
If funding becomes available, the DOE could issue a university solicitation to consider innovative concepts for 238Pu production. This research would be directed at possible improvements over the long term, but it would not mitigate the need to provide an assured supply of 238Pu in the near term.
In summary, there are many different options that, in principle, could be used to restart domestic production of 238Pu. Given enough time and money, many approaches could likely be made to work. But given NASA’s ongoing need for RPSs; given the technical, cost, and schedule uncertainties associated with other approaches; and given the schedule and budgetary constraints that exist, the only timely and practical approaches for restarting domestic production of 238Pu involve the use of the DOE’s ATR and HFIR reactors. These are also the lowest-risk approaches because they rely on proven processes and technologies to a much larger extent than any other option.
The committee believes that it is reasonable to establish 5 kg/year as the goal for domestic production of 238Pu for several reasons:
The NI PEIS established that a production rate of 5 kg/year would meet national needs for 238Pu.
NASA’s need for domestic production of 238Pu through 2028 is on the order of 5 kg/year.
It would be difficult to produce 238Pu at a rate substantially higher than 5 kg/year using existing reactors (i.e., the ATR and HFIR) because of technical factors and because these reactors meet currently subscribed and funded needs by other users.
Even so, over the longer term, the national need for 238Pu could exceed 5 kg/year, and long-term efforts to enhance 238Pu production capabilities should consider the need for higher production rates, perhaps in concert with an assessment of long-term national needs and capabilities for the production of key radionuclides.
FINDING. Domestic Production of 238Pu. There are two viable approaches for reestablishing production of 238Pu, both of which would use facilities at Idaho National Laboratory and Oak Ridge National Laboratory. These are the best options, in terms of cost, schedule, and risk, for producing 238Pu in time to minimize the disruption in NASA’s space science and exploration missions powered by RPSs.
FINDING. Alternate Fuels and Innovative Concepts. Relying on fuels other than 238Pu and/or innovative concepts for producing 238Pu as the baseline for reestablishing domestic production of 238Pu would increase technical risk and substantially delay the production schedule. Nevertheless, research into innovative concepts for producing 238Pu, such as the use of a commercial light-water reactor, may be a worthwhile investment in the long-term future of RPSs.
IMMEDIATE ACTION IS REQUIRED
The DOE’s inability to produce 238Pu and its limited ability to sustain its 238Pu stockpile using foreign sources is inconsistent with NASA’s current plans and future ambitions. Because of the short supply of 238Pu, NASA has baselined future space missions with an RPS that has yet to be flight qualified. In addition, NASA has been making mission-limiting decisions based on the short supply of 238Pu. NASA has been eliminating RPSs as an option for some missions and delaying other missions that require RPSs until more 238Pu becomes available. For example, the New Frontiers 3 Announcement of Opportunity is not open to RPS-powered missions (NASA, 2009). This will likely eliminate from consideration some of the missions described in the report Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity (NRC, 2008)
9
TRIGA [Training, Research, Isotopes, General Atomics] reactors, a class of small nuclear reactors designed and manufactured by General Atomics, are pool-type reactors that can be installed without a containment building, and they are designed for use by scientific institutions and universities for undergraduate and graduate education, private commercial research, nondestructive testing, and isotope production. General Atomics has built TRIGA reactors in a variety of configurations and capabilities, with steady state power levels ranging from 20 kilowatts to 16 megawatts (GA, 2009).
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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration
TABLE 3.2 Best-Case Estimate of 238Pu Shortfall through 2028: 238Pu Demand Versus Supply Subsequent to Launch of Outer Planets Flagship 1
Mission
238Pu (kg)
Discovery 14
3.5
New Frontiers 4
5.3
Pressurized Rover 1
14.0
ATHLETE Rover
14.0
New Frontiers 5
1.8-5.3
Discovery 16
3.5
Pressurized Rover 2
14.0
Outer Planets Flagship 2
5.3-6.2
Pressurized Rover 3
14.0
75.4-79.8
Total 238Pu demand subsequent to OPF1
−13.0
Remaining inventory of 238Pu after OPF1 (with ASRGs)
62.4-66.8
Best-case estimate of 238Pu production needed
–58.0
Total 238Pu production if work starts in FY 2010
4.4-8.8
Best-case estimate of 238Pu shortfall
NOTE: ATHLETE, All-Terrain Hex-Legged Extra-Terrestrial Explorer; FY, fiscal year; OPF, Outer Planets Flagship.
because solar power is not feasible for some of the missions described in that report.
The report The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics (NRC, 2003) describes the solar probe mission as the highest priority in the large mission category, with implementation recommended as soon as possible. The Solar Probe mission, now scheduled for launch in 2015, has been rescoped to eliminate the need for an RPS. The rescoped mission will spend more time near the Sun, but the closest point of approach will be 8.5 solar radii from the surface of the Sun instead of 3 (JHU, 2008).
Similar considerations affect other missions. The mission planning teams for OPF 1 have been directed to minimize power and consider the use of ASRGs. The use of a mixed package of RPSs has also been considered. For example, MMRTGs could be used to provide a basic level of power, and ASRGs could be used for additional power for full mission capability. For the OPF 1 mission, concurrent science operations will have to be limited unless there are at least 4 or 5 MMRTGs (or the equivalent number of ASRGs).
The decadal survey for solar and space physics identifies the interstellar probe as another high-priority mission, although it has been deferred until necessary propulsion capabilities are available (NRC, 2003; 2004). Given the demise of Project Prometheus (NASA’s space nuclear reactor power and propulsion program), the interstellar probe is not possible without RPSs (which are far less expensive than space nuclear reactors).
The DOE’s budget does not currently include funds to reestablish production of 238Pu. Yet, even if funding does become available in fiscal year (FY) 2010, full-scale production of 238Pu (5 kg/year) is unlikely to be possible until 2018, and that will be too late to meet all of NASA’s needs. In fact, if the OPF 1 mission uses MMRTGs, as is currently baselined, even if the DOE starts work immediately to restore its 238Pu production capability, there will be a substantial shortfall in meeting NASA’s needs for 238Pu through 2028.
While it remains to be seen whether ASRGs can and will be flight qualified in time for OPF 1, if ASRGs can be used, NASA estimates that there will be 13 kg of 238Pu left from the available stockpile (including future deliveries of Russian 238Pu) to power missions after OPF 1. Those missions (through 2028) and their demand for 238Pu are listed in Table 3.2. They will require a total of 75.4 to 79.8 kg of 238Pu. Thus, the required production from now through FY 2028 is at least 62.4 to 66.8 kg.
Assuming that the DOE begins work in FY 2010 to establish the capability to produce 5 kg of 238Pu per year, it will be able to produce 1 kg of 238Pu in 2016, 2 kg in 2017, and 5 kg in 2018 and in each year thereafter. This amounts to a total production of 58 kg through the end of FY 2028. The net result is a shortfall of 4.4 to 8.8 kg. Thus, even in a “best-case” scenario that minimizes 238Pu demands and maximizes 238Pu supply—which is to say, even if it is optimistically assumed that (1) NASA’s future RPS mission set is limited to those missions listed in the NASA administrator’s letter of April 2008,10 (2) the 238Pu required by each mission is the smallest amount listed in that letter (for missions with a demand for 238Pu that is listed as a range of values), (3) ASRGs are flight qualified in time to use them instead of MMRTGs on OPF 1, and (4) funds for 238Pu production are included in the DOE’s budget for FY 2010—it would not be possible for the DOE to meet NASA’s total demand for 238Pu. Immediate action is required to minimize the mismatch between NASA needs and the DOE capabilities and to avoid
10
Letter from the NASA administrator Michael D. Griffin to secretary of energy Samuel D. Bodman, April 29, 2008 (reprinted in Appendix C).
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a potential hiatus in U.S. capability to launch RPS-powered spacecraft. Continued inaction will force NASA to make additional, difficult decisions to reduce the science return of some missions and to postpone or eliminate other missions until a source of 238Pu is available.
It has long been recognized that the United States would need to restart domestic production of 238Pu in order to continue producing RPSs. The problem is that the United States has delayed taking action to the point where the situation has become critical, and the dwindling inventory of 238Pu—and uncertainty about the future supply of 238Pu—is now a major constraint on planning the future of the U.S. space program. In recent years, each time a proposal has been made to restore production of 238Pu, action has been deferred. However, the day of reckoning has arrived, and continued delays in taking action to reestablish domestic production of 238Pu will exacerbate the effect of current shortfalls, as detailed in Figure 3.1.
The top part of Figure 3.1 shows three options for future 238Pu supply: (1) funding for 238Pu production is included in
FIGURE 3.1 Potential 238Pu supply, demand, and net balance, 2008 through 2028.
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the DOE’s FY 2010 budget (red line [square data points]), (2) funding for 238Pu production is included in the DOE’s FY 2012 budget (orange line [triangular data points]), or (3) no 238Pu production (black line [circular data points]).
The middle part of Figure 3.1 shows two options for future 238Pu demand: (1) OPF 1 uses MMRTGs (green line [square data points]) or (2) OPF 1 uses ASRGs (blue line [triangular data points]). This plot assumes that 238Pu must be available 1or 2 years before a mission launch date. It also assumes that missions are launched in accordance with the administrator’s letter. Of course, mission launch dates are always subject to change. For example, the best estimate for the OPF 1 launch date is now 2020, not 2017 as indicated in the administrator’s letter. Although changes such as this will change the shape of the middle portion of the demand and balance curves, they do not change the end result, which is that NASA is facing a shortfall in 238Pu that will be difficult to overcome.
The bottom part of Figure 3.1 shows the future 238Pu balance for several combinations of 238Pu supply and demand. The blue lines [triangular data points] depict combinations where OPF 1 uses ASRGs. The green lines [square data points] depict combinations where OPF 1 uses MMRTGs. Every possible combination of 238Pu supply and demand, including those not shown in the figure, results in a future shortfall of 238Pu.
A continuation of the status quo (no production of 238Pu and OPF 1 uses MMRTGs) results in the largest shortfall, with all available 238Pu consumed by 2019. The best-case scenario has the smallest shortfall. However, it seems unlikely that all of the assumptions that are built into the best-case scenario will come to pass. MMRTGs are still baselined for OPF 1, there remains no clear path to fight qualification of ASRGs, and FY 2010 funding for 238Pu production remains more of a hope than an expectation. Thus, the actual shortfall is likely to fall somewhere between the best-case curve and the status-quo curve, and it could easily be 20 kg or more instead of the 4 to 9 kg calculated in Table 3.2.
Continued inaction is also a problem because of schedule requirements. Space science and exploration missions and spacecraft design vary according to the type of power systems available for use. Mission planners require assurance, early in the planning process, that the 238Pu required by a prospective mission will be there when it is needed. All available 238Pu will be essentially consumed by the Mars Science Laboratory, Discovery 12, and OPF 1 missions (assuming MMRTGs are used for OPF 1, in accordance with NASA’s current plans). NASA is unlikely to initiate competitive procurements or develop additional RPS-powered spacecraft until the DOE begins construction of the facilities required to produce the 238Pu needed by those additional missions. As shown in Figure 3.2, if the DOE receives funding in FY 2010 for 238Pu production, the DOE should be able to begin construction of new facilities and/or modification of existing facilities, as necessary, by the end of FY 2013, which would enable the next set of RPS-powered missions (Discovery 14, New Frontiers 4, and the first pressurized lunar rover) to proceed on schedule. However, a delay of one year could force a delay in the New Frontiers 4 schedule, and delay of two years or more could force a delay in the schedule of Discovery 14, the first lunar rover, and subsequent missions.
FINDING. Current Impact. NASA has already been making mission-limiting decisions based on the short supply of 238Pu.
FINDING. Urgency. Even if the Department of Energy budget for fiscal year 2010 includes funds for reestablishing 238Pu production, some of NASA’s future demand for 238Pu will not be met. Continued delays will increase the shortfall.
HIGH-PRIORITY RECOMMENDATION. Plutonium-238 Production. The fiscal year 2010 federal budget should fund the Department of Energy (DOE) to reestablish production of 238Pu.
As soon as possible, the DOE and the Office of Management and Budget should request—and Congress should provide—adequate funds to produce 5 kg of 238Pu per year.
NASA should issue annual letters to the DOE defining the future demand for 238Pu.
RPS MISSION LAUNCH RATE
Late in the study process—after the committee had completed all scheduled meetings—a new issue was raised about the DOE’s ability to support the high launch rate for future RPS missions that NASA currently anticipates.
The United States has launched a total of 26 RPS missions since 1961, but only 4 have been launched since 1977 (Galileo, Ulysses, Cassini, and Pluto/New Horizons). The NASA administrator’s letter of April 2008 anticipates 12 RPS missions in the next 20 years, with 9 of those missions launched during the 9-year period ending in 2028.11 Current DOE facilities used for fueling, processing, testing, and shipping RPS units—as well as the DOE workforce needed to conduct radiological contingency planning—can accommodate the relatively low RPS launch rate of recent decades, but some improvements may be needed to accommodate a sustained launch rate of one mission per year.
To address this issue comprehensively, it would be useful to identify all constraints that the DOE and NASA must overcome to increase the launch rate for RPS missions, and how those constraints could be overcome. Relevant information would include a comparison of historic and future launch rates for space nuclear systems and missions. For example,
11
Letter from the NASA administrator Michael D. Griffin to secretary of energy Samuel D. Bodman, April 29, 2008 (reprinted in Appendix C).
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FIGURE 3.2 Time line for reestablishing domestic 238Pu production and NASA mission planning, 2010 through 2028, assuming the Department of Energy starts work in fiscal year 2010.
15 RPS missions were launched during a period of 8½ years from April 1969 through September 1977. Those missions included 31 RPSs of four different designs (see Table 2.1). It would be useful to know what it took to accomplish this feat in terms of staff, facilities, and facility usage at the DOE and at NASA, especially at the Jet Propulsion Laboratory and the Kennedy Space Center.
Assessments of workforce issues related to radiological contingency planning associated with the Safety Review and Launch Approval Process under Presidential Directive/National Security Council Memorandum 25 (PD/NSC-25, 1977) should also consider the demands of additional missions that use radioisotope heater units but not RPSs (e.g., the Mars Pathfinder mission and the Mars Exploration Rover A and B missions).12 Also, not all launch reviews are equal.
12
Radioisotope heater units (RHUs) provide small amounts of heat (on the order of 1 W) to keep selected spacecraft components warm. They are used when mass and electrical power are at a premium for providing spacecraft thermal control. RHUs produce heat from the natural decay of radioactive material, but they do not produce electricity.
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Although Galileo and Ulysses were launched one year apart, and even though both used the same launch system and the same RPS design, the Ulysses review was just as involved as the Galileo review because the Ulysses GPHS-RTG was oriented 90 degrees from those on the Galileo spacecraft. In contrast, for the Apollo missions the first safety review was exhaustive, but subsequent Apollo safety reviews were abbreviated, focusing on mission and system differences. Pioneer 10 and 11 were reviewed together, as were Viking 1 and 2, LES 8 and 9, and Voyager 1 and 2.
Although the committee did not have the time or information necessary to assess launch rate issues, the committee is confident that the short supply of 238Pu is by far the most urgent issue that must be addressed to carry out NASA’s plans for RPS missions. Still, a detailed investigation of launch rate issues would be advisable because inattention could eventually allow them to become a mission-limiting factor.
REFERENCES
DOE (Department of Energy). 2000. Final Programmatic Environmental Impact Statement for Accomplishing Expanded Civilian Nuclear Energy Research and Development and Isotope Production Missions in the United States, Including the Role of the Fast Flux Test Facility. DOE/EIS-0310. December 2000. Washington, D.C.: U.S. Department of Energy.
DOE. 2001. Record of Decision for the Programmatic Environmental Impact Statement for Accomplishing Expanded Civilian Nuclear Energy Research and Development and Isotope Production Missions in the United States, Including the Role of the Fast Flux Test Facility. Federal Register 66(18): 7877-7887. Available at http://www.epa.gov/EPA-IMPACT/2001/January/Day-26/i2271.htm.
GA (General Atomics). 2009. TRIGA Research Reactors. Available at http://triga.ga.com/45years.html.
JHU (Johns Hopkins University). 2008. Solar Probe+ Mission Engineering Study Report. Laurel, Md.: Johns Hopkins University Applied Physics Laboratory.
NASA (National Aeronautics and Space Administration). Announcement of Opportunity: New Frontiers 2009. NNH09ZDA007O. Release date April 20, 2009.
NEPA (National Environmental Policy Act). 1970. National Environmental Policy Act of 1969, as amended, 42 USC Sections 4321-4347. Available at http://ceq.hss.doe.gov/Nepa/regs/nepa/nepaeqia.htm.
NRC (National Research Council). 2003. The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics. Washington, D.C.: The National Academies Press.
NRC. 2004. Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report. Washington, D.C.: The National Academies Press.
NRC. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, D.C.: The National Academies Press.
PD/NSC-25 (Presidential Directive/National Security Council-25). 1977. Scientific or Technological Experiments with Possible Large-Scale Adverse Environmental Effects and Launch of Nuclear Systems into Space. December 14, 1977 (as amended).